The present invention relates to a method of predicting the location of a high frequency emitter based on observation collected regarding the signal propagated by the emitter.
High frequency (HF) signals, those signals between about two and about thirty megahertz, can be used to communicate various forms of data over long distances. HF broadcasting systems have a variety of uses in commercial, governmental, and defense situations and are particularly useful for providing communication in areas lacking the infrastructure that would allow for more conventional communication methods. Thus, HF broadcasting systems are used by people in remote areas of the world who do not have access to telephone service, and are used by landowners who own large tracts of land and who wish to communicate across these large expanses.
Over the last thirty to forty years, tremendous resources have been dedicated to understanding and predicting the propagation of HF signals throughout the world. One source of innovation regarding signal propagation prediction is the Voice of America, an international broadcasting service supported by the U.S. government. Launched in 1942, the goal of the Voice of America has been to broadcast news to all areas of the world, especially to closed and war torn areas. In working towards this goal, the Voice of America has collected mass amounts of data regarding signal propagation. This data was used by the U.S. Naval Research Laboratory and the Institute for Telecommunication Sciences of the Department of Commerce to develop the Voice of America Coverage Analysis Program (VOACAP).
VOACAP is a powerful tool for predicting the expected performance of HF broadcasting systems. VOACAP is capable of point to point predictions, area coverage predications, and signal to noise to interference calculations. With the advent of modern computers having increased computing power and speed, it has become possible to make the technology of VOACAP more widely available. The current version of VOACAP is available in a Windows format and can be downloaded for free over the Internet. The source code for VOACAP is likewise available for free over the Internet.
Though a very powerful tool for analyzing signal propagation, VOACAP is also very difficult for any but the most experienced user to use. More recently, other software packages have been developed which organize and interrelate years of collected data in a useful manner to aid in signal propagation prediction. One such example is PropMan2000(trademark), available from Rockwell Collins, which includes a graphical user interface to VOACAP in an attempt to make VOACAP easier to use.
Signal prediction software, such as VOACAP and Propman, is typically capable of many types of propagation analysis. HF system performance is dependent on a complex relationship of many factors, the most important of which include the time of day, solar conditions, the frequency used, and the signal to noise ratio of the signal. When using the software, it is possible to predict the area of coverage the signal will have based on these factors and certain other characteristics of the signal, including the type of antenna, the strength or power of the signal, and the location. It is also possible to graphically represent the signal to noise ratio and other propagation parameters.
Though there are many tools available for mapping the coverage area of a given signal, there are fewer tools available for determining the location of the an emitter which is broadcasting an HF signal. Currently, when attempting to determine the location of a HF emitter, several vertical antennas must be positioned in a large ring for receiving signals from the emitter. Such a system requires real hardware, must be done in real time and with real data. It is not only time consuming, but also very expensive given the hardware requirements.
Thus, there is a need in the art for a method of locating a signal emitter which is inexpensive and which does not require extensive hardware.
The present invention is a method of determining a location of an emitter based on observations of the signal being broadcast by the emitter. If the location of an emitter is unknown, several observations of the broadcast signal are collected. The observations may be collected using several stations positioned throughout the globe, or may be collected at a single observation station. Each time the signal is observed, a parameter of the signal, such as the observed signal to noise ratio, is recorded. Based on the location of the observation station, the frequency of the observed signal, the time at which the signal is observed, and the observed signal to noise ratio, it is possible to generate location data indicating a likely location of the emitter. This data is generated for each observation. Next, the location data from all of the observations is compared to determine areas of overlap. An area of intersection where location data from each observation overlaps is the likely location of the emitter.